They say that eyes are the windows to the world. But what if these very windows stop working? Retinitis Pigmentosa is a group of rare genetic disorders that involve the breakdown and loss of cells in the retina (light-sensitive tissue that lines the back of the eye). It affects the retina’s ability to respond to light, causing a slow or progressive loss of vision.
Retinitis Pigmentosa (RP) is considered to be rare. It is generally estimated that this disorder affects about 1 in 3,000 to 1 in 4,000 people in the world.
CAUSES:
It is an inherited disorder that results from damage of any one of more than 50 genes. These genes carry the instructions for making proteins that are needed in cells in the retina called photoreceptors (these are cells in the retina that absorb and convert light into electrical signals). There can be three major causes or cases:
Mutations (change in the base sequence of DNA) within the gene, that are so severe that the gene cannot make the required protein, limiting the cell’s function.
Other mutations can produce a protein that is toxic to the cell, hence the cell cannot function properly.
Other mutations can also lead to the production of an abnormal protein that does not function properly.
How is RP inherited? The mechanism behind inheritance
Genes are short lengths of DNA that code for a protein and are bundled together on structures called chromosomes. Each cell in the body contains 23 pairs of chromosomes. One copy of each chromosome is passed by a parent at conception through egg and sperm cells. The X and Y chromosomes or the sex chromosomes determine whether a person is born a male (XY) or female (XX). The 22 other paired chromosomes are called autosomes and contain a majority of genes that determine non- sex traits. There are three ways in which RP can be inherited:
Autosomal recessive inheritance (it takes two copies of the mutant gene to give rise to the disorder)
Autosomal dominant inheritance (it takes just one copy of the gene to bring about the disorder)
X-linked inheritance (mothers carry the mutated on one of their X chromosomes and pass it down)
SYMPTOMS:
People first experience night blindness and a progressive loss of the visual field. In the late stages, people experience tunnel vision and have difficulty performing essential tasks like reading, walking, driving and recognizing faces and places. People with RP may also develop photophobia (fear of bright lights). Symptoms generally began in childhood with children tripping and finding it difficult to move in the dark. However, the progression of these symptoms varies greatly from person to person. Some retain their central vision even in the late stages and some lose sight immediately.
Diagnostic testing:
Abnormal, dark pigment deposits that streak the retina are a good indication of this disorder. Some tests are:
Electroretinogram (ERG): this measures the electrical activity of photoreceptor cells. People with RP have decreased electrical activity.
Visual field testing: this is to determine the extent of a person’s vision loss.
Genetic testing: a DNA sample is taken from the person to give a genetic diagnosis.
TREATMENT:
There are many devices that can help with living with vision loss. Especially for children, to maximise existing vision, there are special lenses that magnify central vision to expand the visual field and eliminate glare. There are computer programs that can help and many mobility programs and therapists.
Targeted therapies: a clinical trial found that a daily dose of 15,000 international units of vitamin A palmitate slowed the progression of the disorder in adults. An artificial device called the Argus II has also shown promise for restoring some vision for those with late-stage RP. It is a prosthetic device that consists of a light-sensitive electrode that is surgically implanted on the retina.
Many clinical trials are ongoing and gene therapies are being tested to find a promising cure for this disorder. Some of these trials include testing the long term benefits of stem cells (undifferentiated cells which can be made to retinal cells) as well.
Written by: Svasti Tewari
Edited by: Sakshi Deshpande
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